This invention relates to a method for providing improved heat transfer fluids, particularly for use in low temperature environments.
The use of chemical additives as hydraulic drag reducing agents to increase liquid and slurry flows through pipelines has been widely studied. However, such studies make no reference to applications at temperatures appreciably below ambient, which are of great interest in space cooling and other refrigerative systems employing chilled water and ice slurry flows. For such applications, temperatures in the range of -5 to +15.degree. C. are of particular interest. Compatibility with polyhydric alcohols and other solutes may also be required.
Ohlendorf et al, Canadian Patent No. 1,231,099, issued Jan. 5, 1988 describes the use of n-dodecyl(lauryl)-trimethylammonium salicylate as a drag reducing agent for temperatures in the range of 0.degree.-30.degree. C. at dosages of 5-25 mM. However, in attempts to reproduce the Ohlendorf work using 5-10 mM solutions in both water and 10% w/w ethylene glycol at temperatures from ambient to -5.degree. C., the above additive was found to be reasonably effective in the range of +10.degree. to +20.degree. C., but was ineffective at lower temperatures. Transient supercooling was observed when these solutions were cooled to 5.degree. C. and below, with the result that significant drag reduction was observed over several rheometer test cycles but precipitation eventually occurred and drag reduction ceased. It appears that the n-dodecyl(lauryl)trimethylammonium salicylate has a Krafft point (minimum temperature at which the solubility of its monomeric form exceeds its critical micellar concentration value) of close to 5.degree. C. A necessary but insufficient requirement for drag reducing capability is that the monomeric form of the additive be sufficiently soluble for its concentration in solution to reach the minimum level necessary for colloidal micelle formation, mainly the critical micellar concentration. A second requirement is that the additive should be capable of forming asymmetric, "rod-like" micelles.
Recognized drag reducing agents of the type represented by n-dodecyl(lauryl)trimethylammonium salicylate consists of two components, an n-alkyl trimethylammonium cationic surfactant and an anionic counterion which aids micelle formation by reducing electrostatic repulsion between neighbouring surfactant molecules in the micelle. As has been indicated above, the ability of these surface active compounds to act as hydraulic drag reducing agents is associated with their capacity to form asymmetric, rod-like colloidal micelles. Asymmetric micelle formation is achieved only over the temperature range between the Krafft point and the micellar transformation temperature for the given additive and dosage level.
Each of the two additive constituents, the surfactant and the counterion, must be chosen to ensure that the Krafft point of the additive pair lies below the lower limit of the desired operating temperature range. This requirement is relatively easy to meet for operating temperatures above ambient, but is not met by any of the additives thus far recommend in the literature when employed at temperatures close to and below the normal freezing point of water. For this below-ambient temperature range, the reduced solubility of the monomeric form of the additive greatly restricts the choice of candidate compounds.
The Krafft point of a given additive system is defined by the intersection between its monomer solubility vs. temperature relationship and the relationship between solution temperature and the minimum additive concentration necessary for micelle formation to occur (the cmc value). The Krafft point may be reduced by modifying the composition of the additive so as to raise its solubility and/or reduce its cmc value. However, in any given homologous series of additives, higher solubility is associated with higher cmc values.
Homologs of n-dodecyl(lauryl)triammonium salicylate containing longer alkyl chains are more amphiphilic, and therefore exhibit stronger micelle forming tendencies and hence lower cmc values. However, the higher homologs of n-dodecyl(lauryl)triammonium salicylate are less soluble in water, and findings of the present inventors indicate that the effect of lower solubility in elevating the Krafft point more than compensates for the effect of lower cmc values of depressing it. Thus, the net effect is an increase in Krafft point with increase in alkyl chain length.
Schmitt et al, U.K. Patent 1,205,721 describes viscosity reducing agents, including 2-, 3- and 4-hydroxybenzoic acids, 3,5-dihydroxybenzoic acid and a wide variety of other compounds. However, under turbulent flow conditions of practical interest, hydraulic drag is virtually independent of solution viscosity. Because drag reducers work by creating long-range order, it is frequently found that counterions which produce the largest increases in viscosity are also the most effective aids to drag reduction. The agents described by Schmitt are not suitable drag reducers, particularly at low temperatures.
Toet et al, U.S. Pat. No. 4,615,825 relates to friction reduction using a viscoelastic surfactant. It shows that the friction exhibited by an aqueous liquid containing an alkyl trimethylammonium surfactant is further reduced by adding o-hydroxybenzoate counterion. However, none of the agents described are useful as drag reducers at low temperatures.
It is the object of the present invention to find counterions which could be used with an n-alkyl trimethylammonium cationic surfactant which would aid micelle formation and so bring down the cmc value. The specific object of the invention is to reduce the Krafft point of the additive system to a target of close to or below the freezing point of water while maintaining a rod-to-sphere transformation temperature of at least 25.degree. C. at a dosage level of 5 mM.